Memory cells and semiconductor devices including ferroelectric materials

ABSTRACT

Methods of operating a ferroelectric memory cell. The method comprises applying one of a positive bias voltage and a negative bias voltage to a ferroelectric memory cell comprising a capacitor including a top electrode, a bottom electrode, a ferroelectric material between the top electrode and the bottom electrode, and an interfacial material between the ferroelectric material and one of the top electrode and the bottom electrode. The method further comprises applying another of the positive bias voltage and the negative bias voltage to the ferroelectric memory cell to switch a polarization of the ferroelectric memory cell, wherein an absolute value of the negative bias voltage is different from an absolute value of the positive bias voltage. Ferroelectric memory cells are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/631,317, filed Jun. 23, 2017, pending, which is a continuation ofU.S. patent application Ser. No. 15/241,550, filed Aug. 19, 2016, nowU.S. Pat. No. 9,697,881, issued Jul. 4, 2017, which is a continuation ofU.S. patent application Ser. No. 14/842,124, filed Sep. 1, 2015, nowU.S. Pat. No. 9,460,770, issued Oct. 4, 2016, the disclosure of each ofwhich is hereby incorporated herein in its entirety by this reference.

TECHNICAL FIELD

Embodiments disclosed herein relate to methods of operatingferroelectric memory cells including ferroelectric materials exhibitingasymmetric ferroelectric properties, and to such ferroelectric memorycells.

BACKGROUND

Ferroelectric random-access memory (FeRAM) cells have been consideredfor use in many memory arrays. FeRAM cells include a ferroelectricmaterial having a switchable polarization responsive to application ofan electric field (e.g., a bias voltage). The polarization state of theferroelectric material in the FeRAM cell may be used to determine alogic state (e.g., a 1 or a 0) of the FeRAM cell. After the bias voltageis removed, the polarization of the ferroelectric material may remain.The FeRAM cell is therefore, non-volatile, eliminating the need torefresh the memory cell periodically.

Conventional FeRAM cells under an applied field theoretically exhibit asquare hysteresis loop 102, as illustrated in FIG. 1, since atoms of theferroelectric material transition between two equally favorable states.The FeRAM cell is switched from one operational state to anotheroperational state by exposing the FeRAM cell to a switching biasvoltage. For example, the ferroelectric material may be exposed to apositive voltage to switch the polarization of the ferroelectricmaterial to a first direction. At a large enough positive voltage(characterized as the positive switching voltage), the polarization ofthe ferroelectric material switches from a negative polarization to apositive polarization. To switch the FeRAM cell to another state, theferroelectric material is exposed to a negative switching voltage tochange the polarization of the ferroelectric material to a second,opposite direction. Conventionally, the positive switching voltage andthe negative switching voltage applied to a conventional FeRAM cell areequal in magnitude (e.g., have the same absolute value, also referred toherein as a symmetric biasing scheme).

Unfortunately, many FeRAM cells require utilization of a high biasvoltage to switch between different polarization states. Any powersavings realized by the non-volatility of the FeRAM cell relative to aDRAM cell are offset by the high bias voltages that must be applied toswitch the polarization state of the ferroelectric material. Thus,exposing the ferroelectric materials to the higher voltages increasespower consumption of the FeRAM cells, increases operating costs, and mayalso decrease the useful life of the FeRAM cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hysteresis curve during use and operation of a conventionalferroelectric memory cell;

FIG. 2 is a cross-sectional view of an asymmetric ferroelectriccapacitor, in accordance with an embodiment of the present disclosure;

FIG. 3 is a cross-sectional view of a ferroelectric memory cellincluding the asymmetric ferroelectric capacitor of FIG. 2, inaccordance with an embodiment of the present disclosure;

FIG. 4 is a graphical representation of an asymmetric biasing scheme foroperation of a ferroelectric memory cell, in accordance with anembodiment of the present disclosure;

FIG. 5A is a hysteresis curve during use and operation of aferroelectric memory cell in accordance with an embodiment of thepresent disclosure;

FIG. 5B is a graph of a signal strength vs. cycle number of aferroelectric memory cell operated with a symmetric biasing schemecompared to the ferroelectric memory cell operated with an asymmetricbiasing scheme in accordance with an embodiment of the presentdisclosure;

FIG. 5C is a graphical representation of frequency-dependent signal lossduring cycling of a ferroelectric memory cell when the cell is operatedwith a symmetric biasing scheme and an asymmetric biasing scheme at 30°C.;

FIG. 5D is a graphical representation of frequency-dependent signal lossduring cycling of a ferroelectric memory cell when the cell is operatedwith a symmetric biasing scheme and an asymmetric biasing scheme at 100°C.;

FIG. 5E and FIG. 5F are graphs illustrating the voltage and the currentof ferroelectric memory cells operating with a symmetric biasing schemeand an asymmetric biasing scheme, respectively, at various cyclenumbers;

FIG. 6A is a hysteresis curve during use and operation of an asymmetricferroelectric memory cell in accordance with an embodiment of thepresent disclosure;

FIG. 6B is a graph of a signal strength vs. cycle number of aferroelectric memory cell operated with a symmetric biasing schemecompared to the ferroelectric memory cell operated with an asymmetricbiasing scheme in accordance with an embodiment of the presentdisclosure;

FIG. 6C is a graphical representation of frequency-dependent signal lossduring cycling of a ferroelectric memory cell when the cell is operatedwith a symmetric biasing scheme and an asymmetric biasing scheme at 30°C.;

FIG. 6D is a graphical representation of frequency-dependent signal lossduring cycling of a ferroelectric memory cell when the cell is operatedwith a symmetric biasing scheme and an asymmetric biasing scheme at 100°C.;

FIG. 6E is a graphical representation of signal strength as a functionof cycle number for ferroelectric cells operated at a constant negativebias voltage and different positive bias voltages; and

FIG. 6F is a graphical representation of signal strength as a functionof cycle number for ferroelectric cells operated at a constant positivebias voltage and different negative bias voltages.

DETAILED DESCRIPTION

The illustrations included herewith are not meant to be actual views ofany particular systems or semiconductor devices, but are merelyidealized representations that are employed to describe embodimentsherein. Elements and features common between figures may retain the samenumerical designation.

The following description provides specific details, such as materialtypes, material thicknesses, and processing conditions in order toprovide a thorough description of embodiments described herein. However,a person of ordinary skill in the art will understand that theembodiments disclosed herein may be practiced without employing thesespecific details. Indeed, the embodiments may be practiced inconjunction with conventional fabrication techniques employed in thesemiconductor industry. In addition, the description provided hereindoes not form a complete process flow for manufacturing ferroelectricmemory cells, and the ferroelectric memory cells described below do notform a complete ferroelectric memory cell. Only those process acts andstructures necessary to understand the embodiments described herein aredescribed in detail below. Additional acts to form a completeferroelectric memory cell may be performed by conventional techniques.

As used herein, the term “switching voltage” means and includes a biasvoltage applied between a pair of electrodes (e.g., of a capacitor)sufficient to switch a polarization state of a ferroelectric materialdisposed between the pair of electrodes. The bias voltage may be apositive bias voltage, in which case the switching voltage is referredto as a “positive switching voltage,” or the bias voltage may be anegative bias voltage, in which case the switching voltage is referredto as a “negative switching voltage.”

According to some embodiments, a method of operating a ferroelectricmemory cell by applying an asymmetric biasing scheme is disclosed. Theferroelectric memory cell may be asymmetric and may exhibit asymmetricswitching characteristics. As used herein, the term “asymmetricferroelectric memory cell” means and includes a memory cell including aferroelectric material disposed between two electrodes. The asymmetricferroelectric memory cell may include an interfacial material betweenone of the electrodes and the ferroelectric material. In someembodiments, each of the electrodes also has different thicknesses or isformed by different methods.

As used herein, the term “asymmetric biasing scheme” means and includesapplying a bias voltage (e.g., a potential) across the electrodes of aferroelectric memory cell to switch a polarization of the ferroelectricmaterial of the ferroelectric memory cell from a first state to a secondstate that is different than a bias voltage applied across theelectrodes to switch the polarization from the second state to the firststate. In other words, applying an asymmetric biasing scheme includesapplying a positive switching voltage that is different in magnitudethan a negative switching voltage. For example, a direction of apolarization of the ferroelectric memory cell may be switched from afirst direction to a second direction by applying a positive biasvoltage across the ferroelectric memory cell that is different from anegative bias voltage to switch the direction of polarization from thesecond direction to the first direction. Thus, the ferroelectric memorycell may be switched from a first polarization to a second polarizationat a positive bias voltage with a different absolute value than anegative bias voltage to switch from the second polarization state tothe first polarization state. Operating the ferroelectric memory cellwith the asymmetric biasing scheme may reduce the power used to operatethe ferroelectric memory cell and may increase the effective operatinglife of the ferroelectric memory cell. Operating the ferroelectricmemory cell with the asymmetric biasing scheme may also provide a moreconsistent switching signal strength over the lifetime of theferroelectric memory cell at different operating conditions, such as atdifferent frequency pulses.

FIG. 2 illustrates a capacitor 200 including a ferroelectric material206. The capacitor 200 may form a part of a ferroelectric memory cellaccording to embodiments of the disclosure and may include a bottomelectrode 202, an interfacial material 204 overlying the bottomelectrode 202, a ferroelectric material 206 overlying the interfacialmaterial 204, and a top electrode 208 overlying the ferroelectricmaterial. The capacitor 200 may be, for example, a metal-insulator-metal(MIM) capacitor. While the capacitor 200 is described and illustrated asbeing used in ferroelectric memory cells, the capacitor 200 may also beused in dynamic random-access memory (DRAM) applications.

The bottom electrode 202 may include a conductive material. In someembodiments, the bottom electrode 202 includes titanium, titaniumnitride (TiN), titanium aluminum nitride (TiAlN), tantalum nitride(TaN), platinum, combinations thereof, or other conductive materials. Insome embodiments, the bottom electrode 202 may be doped with carbon. Thebottom electrode 202 may be formed by sputtering, atomic layerdeposition (ALD), chemical vapor deposition (CVD), physical vapordeposition (PVD), plasma enhanced chemical vapor deposition (PECVD), lowpressure chemical vapor deposition (LPCVD), or other suitable process.

The interfacial material 204 may directly overlie and contact the bottomelectrode 202 and may intervene between the bottom electrode 202 and theferroelectric material 206. In some embodiments, the interfacialmaterial 204 includes an oxide of the material of the bottom electrode202. For example, where the bottom electrode 202 comprises titaniumnitride, the interfacial material 204 may include titanium oxide(TiO_(x)), such as titanium dioxide (TiO₂). In other embodiments, theinterfacial material 204 may include a non-conductive dielectricmaterial, such as, for example, aluminum nitride (AlN). As will bedescribed herein, the capacitor 200 including the interfacial material204 may form an asymmetric capacitor 200 exhibiting an asymmetrichysteresis loop.

The ferroelectric material 206 may directly overlie and contact theinterfacial material 204. The ferroelectric material 206 may include adielectric material that exhibits a polarization (e.g., a displacementof oppositely charged ions to create a dipole moment) that is switchableby an external electric field. Thus, the ferroelectric material 206 mayinclude a material capable of exhibiting a switchable polarizationresponsive to exposure to a switching voltage. In addition, theferroelectric material 206 may include a remnant polarization (P_(r))that may remain after removing the external field. As a result, thepolarization of the ferroelectric material 206 may be interpreted as thestate (e.g., a 1 or a 0) of the associated memory cell. Theferroelectric material 206 may include one or more of hafnium oxide(HfO_(x)), zirconium oxide (ZrO_(x)), lead zirconate titanate (PZT),another ferroelectric material known in the art, or combinationsthereof. In some embodiments, the ferroelectric material 206 includeshafnium dioxide (HfO₂) or zirconium dioxide (ZrO₂).

The ferroelectric material 206 may include one or more dopants. Forexample, the ferroelectric material 206 may include one or more ofsilicon, aluminum, zirconium, magnesium, strontium, gadolinium, yttrium,other rare earth elements, and combinations thereof.

The top electrode 208 may directly overlie and contact the ferroelectricmaterial 206. The top electrode 208 may include a conductive material.In some embodiments, the top electrode 208 includes titanium, titaniumnitride, titanium aluminum nitride, tantalum nitride, platinum,combinations thereof, or other conductive materials. The top electrode208 may be formed by sputtering, atomic layer deposition, chemical vapordeposition, physical vapor deposition, plasma enhanced chemical vapordeposition, low pressure chemical vapor deposition, or other suitableprocess.

In some embodiments, the top electrode 208 includes a material that isdifferent than the bottom electrode 202. In other embodiments, the topelectrode 208 may have a different thickness than the bottom electrode202. In yet other embodiments, the top electrode 208 may be formed by adifferent method (e.g., ALD) than the bottom electrode 202. A topelectrode 208 that includes a material different than the bottomelectrode 202, has a thickness that is different than a thickness of thebottom electrode 202, is formed by a different method than the bottomelectrode 202, or combinations thereof, may form an asymmetric capacitor200.

In some embodiments, the capacitor 200 comprises a bottom electrode 202including titanium aluminum nitride, an interfacial material 204including aluminum nitride, a ferroelectric material 206 including oneor more of hafnium oxide and zirconium oxide, and a top electrode 208including titanium nitride. In other embodiments, the capacitor 200comprises a bottom electrode 202 including titanium nitride, aninterfacial material 204 including titanium oxide, a ferroelectricmaterial 206 including one or more of hafnium oxide and zirconium oxide,and a top electrode 208 including titanium nitride.

Although FIG. 2 illustrates the interfacial material 204 as beingdisposed directly between the bottom electrode 202 and the ferroelectricmaterial 206, the interfacial material 204 may be between theferroelectric material 206 and the top electrode 208. In some suchembodiments, the ferroelectric material 206 may directly overlie andcontact the bottom electrode 202. In some embodiments, the capacitor 200includes only one interfacial material 204 disposed between either thebottom electrode 202 and the ferroelectric material 206 or between theferroelectric material 206 and the top electrode 208 (i.e., theinterfacial material 204 may be located on only one side of theferroelectric material 206). It is contemplated that, in otherembodiments, the capacitor 200 may include an interfacial material 204between the bottom electrode 202 and the ferroelectric material 206 andanother interfacial material 204 between the top electrode 208 and theferroelectric material 206. In some such embodiments, the interfacialmaterial 204 between the top electrode 208 and the ferroelectricmaterial 206 may be formed of a different material or may have adifferent thickness than the interfacial material 204 between the bottomelectrode 202 and the ferroelectric material 206.

Referring to FIG. 3, a ferroelectric memory cell 300 including thecapacitor 200 is shown. The ferroelectric memory cell 300 includes asubstrate 310 and a source region 314 and a drain region 312 formedwithin the substrate 310. The substrate 310 may be a semiconductorsubstrate, a base semiconductor material on a supporting substrate, ametal electrode, or a semiconductor substrate having one or morematerials, structures, or regions formed thereon. The substrate 310 maybe a conventional silicon substrate or other bulk substrate includingsemiconductor material. As used herein, the term “bulk substrate” meansand includes not only silicon wafers, but also silicon-on-insulator(“SOI”) substrates, such as silicon-on-sapphire (“SOS”) substrates orsilicon-on-glass (“SOG”) substrates, epitaxial layers of silicon on abase semiconductor foundation, or other semiconductor or optoelectronicmaterials, such as silicon-germanium (Si_(1-x)Ge_(x), where x is, forexample, a mole fraction between 0.2 and 0.8), germanium (Ge), galliumarsenide (GaAs), gallium nitride (GaN), or indium phosphide (InP), amongothers. Furthermore, when reference is made to a “substrate” in thefollowing description, previous process stages may have been utilized toform material, regions, or junctions in the base semiconductor structureor foundation.

The ferroelectric memory cell 300 may include an access transistorincluding a dielectric material 316 and a gate electrode 318. Thecapacitor 200 may be connected to the drain region 312 of the transistorvia a conductive contact (e.g., a conductive plug) 320. The conductivecontact 320 may overlie the drain region 312 and may directly contactthe bottom electrode 202 of the capacitor 200. The conductive contact320 may include a conductive material, such as, for example, tungsten,titanium, aluminum, copper, polysilicon, or other suitable conductivematerial.

The gate dielectric material 316 may include a suitable dielectricmaterial. In some embodiments, the gate dielectric material 316 includessilicon dioxide, or a high-k dielectric material such as zirconiumoxide, hafnium oxide, aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃), orother high-k dielectrics known in the art. The source region 314 and thedrain region 312 may be located on opposing sides of the gate dielectricmaterial 316.

The gate electrode 318 may include a conductive material, such as, forexample, titanium, tantalum, tungsten, ruthenium, nitrides thereof,polysilicon, or other suitable conductive gate electrode material.

Accordingly, in one embodiment a ferroelectric memory cell comprises acapacitor overlying a conductive material in contact with at least oneof a source region and a drain region of a semiconductor substrate, thecapacitor comprising a first electrode comprising titanium aluminumnitride, a ferroelectric material comprising hafnium oxide, zirconiumoxide, or a combination thereof, an interfacial material between thefirst electrode and the ferroelectric material, and a second electrodecomprising titanium nitride over the ferroelectric material.

During use and operation, a bias (e.g., the positive switching voltageor the negative switching voltage) may be applied to the ferroelectricmemory cell 300 including the ferroelectric material 206 to switch thepolarization of the ferroelectric material between a first state and asecond state. For example, a potential may be applied between the topelectrode 208 and the bottom electrode 202 to create a potential acrossthe capacitor 200. In some embodiments, the top electrode 208 may beexposed to a positive or negative voltage while the bottom electrode 202is exposed to a zero voltage. In other embodiments, a first voltage maybe applied to the top electrode 208 and a second voltage may be appliedto the bottom electrode 202 such that a difference between the firstvoltage and the second voltage is equal to one of the positive switchingvoltage or the negative switching voltage.

Referring to FIG. 4, an asymmetric biasing scheme for transitioning thepolarization of the ferroelectric memory cell 300 is shown. A first biasvoltage (e.g., the positive switching voltage), such as, for example,about 1.8V, shown at 400, may be applied to the ferroelectric memorycell 300. Responsive to the first bias voltage, the ferroelectricmaterial 206 of the capacitor 200 may become polarized in a firstdirection. After a period of time, the first bias voltage 400 may beremoved (e.g., the ferroelectric memory cell may be exposed to a zerobias), as shown at 402. Responsive to removing the first bias voltage400, the ferroelectric material 206 may return to a remnant polarizationthat may correspond to a logic state of the ferroelectric memory cell300. To switch the polarization of the ferroelectric material 206, asecond bias voltage (e.g., the negative switching voltage) 404, such as,for example, about −1.0V, may be applied to the ferroelectric material206. Thus, an absolute value of the negative switching voltage isdifferent than an absolute value of the positive switching voltage.Responsive to exposure to the second bias voltage 404, the ferroelectricmaterial 206 may be polarized in a second direction, opposite to thefirst direction. After exposing the ferroelectric material 206 to thesecond bias voltage 404, the second bias voltage 404 may be removed andthe ferroelectric material 206 may return to a remnant polarization thatmay correspond to another logic state of the ferroelectric memory cell300.

Although FIG. 4 illustrates a positive bias voltage of about 1.8V and anegative bias voltage of about −1.0V, any asymmetric biasing scheme inwhich an absolute value of the positive bias voltage is different froman absolute value of the negative bias voltage may be used. In someembodiments, the absolute value of one of the positive bias voltage andthe negative bias voltage may be equal to between about twenty-fivepercent and about ninety-nine percent, such as between about twenty-fivepercent and about forty percent, between about forty percent and aboutfifty percent, between about fifty percent and about sixty percent,between about sixty percent and about seventy-five percent, betweenabout seventy-five percent and about ninety percent, or between aboutninety percent and about ninety nine percent of an absolute value of theother of the positive bias voltage and the negative bias voltage. Insome embodiments, an absolute value of one of the positive bias voltageand the negative bias voltage may be less than about two-thirds, such asbetween about two-thirds and about one-half an absolute value of theother of the positive bias voltage and the negative bias voltage.

The first bias voltage 400 and the second bias voltage 404 may beapplied by, for example, applying a potential across the capacitor 200.For example, a first potential (e.g., the positive switching voltage)may be applied between the bottom electrode 202 and the top electrode208 to create a potential across the capacitor 200 and induce apolarization of the ferroelectric material 206 within the capacitor 200.To induce an opposite polarization of the ferroelectric material 206,the second bias voltage 404 may be applied to the ferroelectric material206 by, for example, applying a second potential (e.g., the negativeswitching voltage) between the bottom electrode 202 and the topelectrode 208.

Although FIG. 4 illustrates the use of one form of an asymmetric biasingscheme to induce a transition from one polarization to anotherpolarization, it is contemplated that the polarization may be switchedwith other waveforms, such as, for example, a square pulse or atriangular pulse.

A ferroelectric memory cell including an asymmetric capacitor 200 (FIG.2) having a bottom electrode 202 including titanium nitride, aninterfacial material 204 including titanium oxide, a ferroelectricmaterial 206 including one of zirconium oxide, hafnium oxide, andcombinations thereof, and a top electrode 208 including titanium nitridewas formed. The bottom electrode 202 had a thickness of about 100 Å, theinterfacial material 204 had a thickness of about 5 Å, the ferroelectricmaterial 206 had a thickness of about 70 Å, and the top electrode 208had a thickness of about 50 Å. Performance for such a ferroelectricmemory cell was determined by conventional techniques as illustrated inFIG. 5A through FIG. 5E.

FIG. 5A illustrates a hysteresis curve 500 for such a ferroelectricmemory cell to which the asymmetric biasing scheme is applied. Theasymmetric biasing scheme may include applying a negative switchingvoltage of about −1.2V to the ferroelectric memory cell, as indicated atarrow 502A. Arrow 502 indicates that a polarization of the ferroelectricmaterial 206 may switch from a positive polarization to a negativepolarization at a negative coercive voltage of about −0.7V, located atan inflection point of the hysteresis curve. When the ferroelectricmaterial 206 is exposed to the negative coercive voltage ofapproximately −0.7V (e.g., during application of the negative switchingvoltage), the ferroelectric material 206 may begin to switch from apositive polarization to a negative polarization. After the negativeswitching voltage is removed, the polarization of the ferroelectricmaterial 206 may return to a negative remnant polarization (e.g.,−P_(r)) of about 7 μC/cm².

The asymmetric biasing scheme may include applying a positive switchingvoltage of about 1.8V to the ferroelectric memory cell, as indicated atarrow 504A. Arrow 504 indicates that a polarization of the ferroelectricmaterial 206 may switch from a negative polarization to a positivepolarization at a positive coercive voltage of about 1.1V. When theferroelectric material 206 is exposed to the positive coercive voltageof approximately 1.1V (e.g., during application of the positiveswitching voltage), the ferroelectric material 206 may begin to switchfrom a negative polarization to a positive polarization. After thepositive switching voltage is removed, the polarization of theferroelectric material 206 may return to a positive remnant polarization(e.g., P_(r)) of about 5 μC/cm². Accordingly, the ferroelectric material206 may exhibit asymmetric switching properties. In other words, anabsolute value of the switching voltage used to switch the polarizationof the ferroelectric material 206 from a first polarization to a secondpolarization is not equal to an absolute value of the switching voltageused to switch the polarization of the ferroelectric material 206 fromthe second polarization to the first polarization. For example, theferroelectric material 206 may be switched from a negative polarizationto a positive polarization by applying a positive switching voltage ofapproximately 1.8V to the ferroelectric material 206 while theferroelectric material 206 may be switched from the positivepolarization to the negative polarization by applying a negativeswitching voltage of approximately −1.2V.

Referring to FIG. 5B, a graph illustrating a difference between apositive remnant polarization and a negative polarization of aferroelectric memory cell including the ferroelectric material 206 overseveral cycles of the ferroelectric memory cell is shown. The x-axisplots the cycle number and the y-axis plots the value of 2P_(r), whichis equal to a polarization difference between the positive polarizationstate and the negative polarization state of the ferroelectric material206. The value of 2P_(r) may be equal to a difference between thepositive remnant polarization and the negative remnant polarization,which, in some embodiments, may correspond to a polarization strength ofthe ferroelectric memory cell including the ferroelectric material. Overthe lifetime of a ferroelectric memory cell, it is desirable for thevalue of 2P_(r) to remain constant so that a constant polarizationsignal may be sensed for reading the logic state of the ferroelectricmemory cell.

With continued reference to FIG. 5B, the upper curve illustrates thepolarization strength over the operating life of the ferroelectricmemory cell while applying a symmetric biasing scheme (e.g., a positiveswitching voltage of about 1.8V and a negative switching voltage ofabout −1.8V). The lower curve illustrates a polarization strength overthe operating life of the same ferroelectric memory cell while applyingan asymmetric biasing scheme (e.g., a positive switching voltage ofabout 1.8V and a negative switching voltage of about −1.2V). Duringinitial stages of operation, and up to about 10⁴ cycles, thepolarization strength with the symmetric biasing scheme and thepolarization strength with the asymmetric biasing scheme aresubstantially flat (e.g., the memory cell exhibits a substantiallyconstant polarization strength), as illustrated at 506 and 510,respectively. However, when operated with the symmetric biasing scheme,the ferroelectric memory cell exhibits an undesirable increased signalpeaking as the number of cycles of the ferroelectric increases, asillustrated at 508. On the other hand, when operated with the asymmetricbiasing scheme, the ferroelectric memory cell exhibits reduced signalpeaking as the number of cycles of the ferroelectric cell increases, asillustrated at 512. Thus, the ferroelectric memory cell may exhibitreduced signal peaking and less variation in signal strength over thecourse of operation of the ferroelectric memory cell when operated withthe asymmetric biasing scheme than with the symmetric biasing scheme.Even though the maximum signal strength is reduced under the asymmetricbiasing scheme, a more constant polarization strength may be preferredfor sensing the operational state of the ferroelectric memory cell.

It is contemplated that one of the positive bias voltage and thenegative bias voltage may be altered during the operating life of theferroelectric memory cell such that the polarization strength ismaintained at a substantially constant strength. In some embodiments,after a predetermined number of cycles, at least one of the positivebias voltage and the negative bias voltage may be adjusted to maintain asubstantially flat polarization strength.

Referring to FIG. 5C, the frequency dependence of the ferroelectricmemory cell as a function of cycle number is shown. The top graph ofFIG. 5C illustrates the read signal of the ferroelectric memory cell asa function of cycle number for cell pulses of different frequencies(e.g., delays of about 50 ns and delays of about 10 μs between pulses)for three different ferroelectric memory cells (labeled as “A,” “B,” and“C”) operated with the symmetric biasing scheme while the bottom graphillustrates the read signal of the ferroelectric memory cell as afunction of cycle number for cell pulses of different frequencies forthree different ferroelectric memory cells operated with the asymmetricbiasing scheme. FIG. 5C illustrates the frequency dependence of theferroelectric memory cells at a temperature of about 30° C. Generally,as the delay time between pulses increases, the read signal undesirablydecreases. The value of 2P_(r)Norm may be defined as the ratio of 2P_(r)with a long delay (e.g., 10 μs) divided by 2P_(r) with a long delay(e.g., 50 ns) after about, for example, 4×10⁷ cycles. In general, it isdesired that the value of 2P_(r)Norm be equal to approximately 1.0,meaning that as the time between cycles is changed (i.e., the cyclefrequency), the read signal of the ferroelectric memory cell does notchange.

The top graph of FIG. 5C illustrates that for the symmetric biasingscheme, the value of 2P_(r)Norm is equal to about 0.833. The bottomgraph of FIG. 5C illustrates that for the asymmetric biasing scheme, thevalue of 2P_(r)Norm is equal to about 0.905. In other words, for theasymmetric biasing scheme, after about 4×10⁷ cycles, the ferroelectricmemory cell exhibits less frequency-dependent signal loss with longerpulses than when operated with the symmetric biasing scheme. Thus, underthe asymmetric biasing scheme, the ferroelectric memory cell exhibitsabout 43% less signal loss than when the ferroelectric memory cell isoperated with the symmetric biasing scheme.

Referring to FIG. 5D, the frequency dependence of the ferroelectricmemory cell as a function of cycle number is shown for a temperature ofabout 100° C. In general, ferroelectric memory cell performance degradesat elevated temperatures due to increased thermal depolarization of theferroelectric material. FIG. 5D illustrates that at 100° C., thefrequency dependence of the ferroelectric memory cells is improved whenoperated with an asymmetric biasing scheme compared to a symmetricbiasing scheme. For example, the value of 2P_(r)Norm for the symmetricbiasing scheme is shown as about 0.539 and the value of 2P_(r)Norm forthe asymmetric biasing scheme is about 0.678. In some embodiments, theferroelectric memory cell may be operated at elevated temperatures,meaning that the improved value of 2P_(r)Norm may be advantageous at theelevated temperatures.

Referring to FIG. 5E, graphs of voltage and current as a function oftime are illustrated for the ferroelectric memory cell operated with asymmetric biasing scheme (e.g., a positive switching voltage of about1.8V and a negative switching voltage of about −1.8V). The voltage andcurrent of the ferroelectric memory cell are plotted after a pluralityof cycle numbers, (e.g., 1×10³ cycles, 1×10⁶ cycles, 1×10⁸ cycles, and1×10¹⁰ cycles). Referring to the graph at the top left, at low cyclecounts (e.g., 1×10³ cycles), the current of the ferroelectric memorycell may exhibit a double peak as indicated at 514. Referring to thegraph at the top right, the double peak 514 may remain after about 1×10⁶cell cycles. The double peak 514 may undesirably cause the ferroelectricmemory cell to switch or may reduce a sensing window of theferroelectric memory cell at the low cycle counts. As an example, theferroelectric memory cell may have a tendency to switch at each of thepeaks of the double peak 514. Referring to the lower graphs of FIG. 5E,the ferroelectric memory cell may no longer exhibit the double peak 514at the 1×10⁸ and 1×10¹⁰ cycles.

Referring to FIG. 5F, graphs of the voltage and current as a function oftime are illustrated for the ferroelectric memory cell operated with anasymmetric biasing scheme. In some embodiments, the asymmetric biasingscheme may include selecting the positive switching voltage to be about1.8V and the negative switching voltage to be about −0.8V. Referring tothe different voltage and current plots, the ferroelectric memory celldoes not exhibit double peaks at either low or high cycle counts.Rather, with reference to the upper graphs (e.g., at 1×10³ and 1×10⁶cycles), only a single peak, indicated as 516, is shown for all of themeasured cycle counts. Accordingly, operating the ferroelectric memorycell with the asymmetric biasing scheme may improve the operation of theferroelectric memory cell and reduce undesired switching of theferroelectric memory cell at low cycle counts.

A ferroelectric memory cell including an asymmetric capacitor 200 (FIG.2) having a bottom electrode 202 including titanium aluminum nitride(TiAlN), a dielectric interfacial material 204 including aluminumnitride (AlN), a ferroelectric material 206 including one of zirconiumoxide, hafnium oxide, and combinations thereof, and a top electrode 208including titanium nitride was formed. The bottom electrode 202 had athickness of about 60 Å, the interfacial material 204 had a thickness ofabout 2 Å, the ferroelectric material 206 had a thickness of about 70 Å,and the top electrode 208 had a thickness of about 50 Å. Performance ofsuch a ferroelectric memory cell was determined by conventionaltechniques as illustrated in FIG. 6A through FIG. 6F.

FIG. 6A illustrates a hysteresis curve 600 for such a ferroelectricmemory cell to which the asymmetric biasing scheme is applied. Theasymmetric biasing scheme may include applying a negative switchingvoltage of about −1.2V to the ferroelectric memory cell, as indicated atarrow 602A. Arrow 602 indicates that a polarization of the ferroelectricmaterial 206 may switch from a positive polarization to a negativepolarization at a negative coercive voltage of about −0.7V, located atan inflection point of the hysteresis curve. When the ferroelectricmaterial 206 is exposed to the negative coercive voltage of about −0.7V(e.g., during application of the negative switching voltage), theferroelectric material 206 may begin to switch from the positivepolarization to the negative polarization. After the negative switchingvoltage is removed, the polarization of the ferroelectric material 206may return to a negative remnant polarization (e.g., −P_(r)) of about−10 μC/cm².

The asymmetric biasing scheme may include applying a positive switchingvoltage of about 1.8V to the ferroelectric memory cell, as indicated atarrow 604A. Arrow 604 indicates that a polarization of the ferroelectricmaterial 206 may switch from a negative polarization to a positivepolarization at a positive coercive voltage of about 1.2V. When theferroelectric material 206 is exposed to the positive coercive voltageof approximately 1.2V (e.g., during application of the positiveswitching voltage), the ferroelectric material 206 may begin to switchfrom a negative polarization to a positive polarization. After removalof the positive switching voltage, the ferroelectric material 206 mayexhibit a positive remnant polarization of about 8 μC/cm². Thus, in someembodiments, the positive remnant polarization and the negative remnantpolarization may have different magnitudes (e.g., an absolute value ofthe positive remnant polarization may not be equal to an absolute valueof the negative remnant polarization).

Accordingly, the ferroelectric material 206 may exhibit asymmetricswitching properties. In other words, an absolute value of the switchingvoltage used to switch the polarization of the ferroelectric material206 from a first polarization to a second polarization is not equal toan absolute value of the switching voltage used to switch thepolarization of the ferroelectric material 206 from the secondpolarization to the first polarization. For example, the ferroelectricmaterial 206 may be switched from a negative polarization to a positivepolarization by applying a positive switching voltage of approximately1.8 to the ferroelectric material 206 while the ferroelectric material206 may be switched from the positive polarization to the negativepolarization by applying a negative switching voltage of about −1.2V.

Referring to FIG. 6B, a graph illustrating a polarization strength ofthe ferroelectric memory cell of FIG. 6A over several cycles of theferroelectric memory cell is shown. The upper curve illustrates thevalue of 2P_(r) of the ferroelectric memory cell while applying asymmetric biasing scheme (e.g., a positive switching voltage of about1.8V and a negative switching voltage of about −1.8V) and the lowercurve illustrates the polarization strength of the ferroelectric memorycell while applying an asymmetric biasing scheme (e.g., a positiveswitching voltage of about 1.8V and a negative switching voltage ofabout −1.2V), as described above with reference to FIG. 5B. Asillustrated at 606 and 610, the polarization strength of theferroelectric memory cell with the symmetric biasing scheme and with theasymmetric biasing scheme are substantially flat during initial stagesof operation. When operated with the symmetric biasing scheme, thepolarization strength begins to increase at about 10⁵ cycles and thesignal peaks at about 10⁸ cycles, as indicated at 608. When operatedwith the asymmetric biasing scheme, the polarization strength begins toincrease at about 10⁶ cycles with the signal peaking occurring at about10⁸ cycles, as indicated at 612. Advantageously, the peak signal at 612is substantially the same as the polarization strength exhibitedthroughout the operating life of the ferroelectric memory cell.Accordingly, over the lifetime of the ferroelectric cell, thepolarization strength of the ferroelectric memory cell operated with theasymmetric biasing scheme may remain substantially constant.

When operated with the symmetric biasing scheme, the ferroelectricmemory cell may begin to fatigue after about 10⁸ cycles. For example,the read signal may begin to decrease after about 10⁸ cycles, and maydecrease to about 6 μC/cm² after about 10¹¹ cycles. When operated withthe asymmetric biasing scheme, the ferroelectric memory cell may notexhibit fatigue as early as when it is operated with the symmetricbiasing scheme. For example, the ferroelectric memory cell may not beginto exhibit fatigue until after about 10⁹ cycles. Thus, when operatedwith the asymmetric biasing scheme, the ferroelectric memory cell mayexhibit a lower amount of signal peaking and may not exhibit fatigueuntil after more operation cycles. When the results of FIG. 6B arecompared to the results of FIG. 5B, which plot the polarization strengthof a ferroelectric memory cell including different materials than thatin FIG. 6B, similar trends were observed.

With continued reference to FIG. 6B, the ferroelectric memory cellincluding the titanium aluminum nitride bottom electrode and thealuminum nitride interfacial material may exhibit less variation in thepolarization strength during operating of the ferroelectric memory cellthan the ferroelectric memory cell including the titanium nitrideelectrodes having different thicknesses.

Referring to FIG. 6C, the frequency dependence of the ferroelectricmemory cell as a function of cycle number at a temperature of about 30°C. is shown. The top graph of FIG. 6C illustrates that for the symmetricbiasing scheme, 2P_(r)Norm is equal to about 0.929. The bottom graphillustrates that for the asymmetric biasing scheme, 2P_(r)Norm is equalto about 0.961. Thus, the ferroelectric memory cell may exhibit lessfrequency-dependent signal loss at longer cycle pulses when operatedwith the asymmetric biasing scheme than when operated with the symmetricbiasing scheme.

Referring to FIG. 6D, the frequency dependence of the ferroelectricmemory cell as a function of cycle number is shown for a temperature ofabout 100° C. The value of 2P_(r)Norm for the symmetric biasing schemeis about 0.759 and the value of 2P_(r)Norm for the asymmetric biasingscheme is about 0.733. Thus, the ferroelectric memory cell may exhibitonly a slightly higher value of 2P_(r)Norm when operated with asymmetric biasing scheme compared to an asymmetric biasing scheme.

Referring to FIG. 6E, the asymmetric biasing scheme may be tailored toachieve a desired signal strength over the operating lifetime of theferroelectric memory cell. FIG. 6E illustrates a plurality of asymmetricbiasing schemes and a symmetric biasing scheme of the asymmetricferroelectric memory cell. Each of the biasing schemes include the samenegative switching voltage (i.e., −1.8V) while changing the positiveswitching voltage. As illustrated in FIG. 6E, the positive switchingvoltage may affect the initial signal level of the ferroelectric memorycell. As the positive switching voltage is increased, the signal levelof the ferroelectric memory cell may also increase.

Referring to FIG. 6F, the asymmetric biasing scheme may be tailored tocontrol the amount of signal peaking and the onset of fatigue. FIG. 6Fillustrates signal strength as a function of cycle number for a numberof biasing schemes having the same positive switching voltage (i.e.,1.8V) while changing the negative switching voltages. In general, whenoperated with negative switching voltages having a larger magnitude(e.g., −2.8V, −2.5V, −2.2V, etc.), the ferroelectric memory cellexhibited a larger amount of undesired signal peaking. However, whenoperated with negative switching voltages having a lower magnitude(e.g., −0.8V, −0.9V, −1.0V, etc.), the ferroelectric memory cellexhibited lower signal strengths and also begins to fatigue at lowercycle numbers. At negative switching voltages such as −1.2V, −1.4V, and−1.6V, the ferroelectric memory cell exhibited substantially flatsignals and did not begin to exhibit fatigue characteristics untilhigher cycle numbers than the other biasing schemes. As one example,when operated with a biasing scheme of a positive switching voltage ofabout 1.8V and a negative switching voltage of about −1.2V, theferroelectric memory cell exhibited a substantially flat signal duringthe operating life of the memory cell and exhibited reduced fatiguecharacteristics, even up until about 10¹⁰ cycles. The asymmetric biasingscheme may, thus, reduce power consumption and maintain desirableperformance. Accordingly, a strong signal may be achieved while alsoreducing the fatigue properties of the ferroelectric memory cell.

Accordingly, in one embodiment, a method of operating a method ofoperating a ferroelectric memory cell comprises applying one of apositive bias voltage and a negative bias voltage to a ferroelectricmemory cell comprising a capacitor including a top electrode, a bottomelectrode, a ferroelectric material between the top electrode and thebottom electrode, and an interfacial material between the ferroelectricmaterial and one of the top electrode and the bottom electrode, andapplying another of the positive bias voltage and the negative biasvoltage to the ferroelectric memory cell to switch a polarization of theferroelectric memory cell, wherein an absolute value of the negativebias voltage is different from an absolute value of the positive biasvoltage.

Accordingly, in another embodiment a method of operating a ferroelectricmemory cell comprises applying one of a positive bias voltage and anegative bias voltage to a ferroelectric capacitor comprising a firstelectrode, an interfacial material between the first electrode and aferroelectric material, and a second electrode adjacent theferroelectric material, and applying another of the positive biasvoltage and the negative bias voltage to the ferroelectric capacitor,the negative bias voltage having a different magnitude than the positivebias voltage.

Operating an asymmetric ferroelectric memory cell with an asymmetricbiasing scheme may reduce power consumption used during operation of theasymmetric ferroelectric memory cell, reduce signal peaking, and reducefrequency-dependent signal loss. Under such an operating scheme, theferroelectric memory cell may not be over-driven and may be configuredto operate for a longer period of time before breaking down. Theferroelectric memory cell may include a top electrode and a bottomelectrode having different thicknesses, formed from different materials,formed by different processing conditions, or combinations thereof. Theferroelectric materials may include hafnium oxide, zirconium oxide, or acombination thereof. An interfacial material may be disposed between theferroelectric material and one of the top electrode and the bottomelectrode.

While certain illustrative embodiments have been described in connectionwith the figures, those of ordinary skill in the art will recognize andappreciate that embodiments encompassed by the disclosure are notlimited to those embodiments explicitly shown and described herein.Rather, many additions, deletions, and modifications to the embodimentsdescribed herein may be made without departing from the scope ofembodiments encompassed by the disclosure, such as those hereinafterclaimed, including legal equivalents. In addition, features from onedisclosed embodiment may be combined with features of another disclosedembodiment while still being encompassed within the scope of thedisclosure as contemplated by the inventors.

What is claimed is:
 1. A semiconductor device, comprising: aferroelectric material comprising hafnium oxide, zirconium oxide, or acombination thereof, the ferroelectric material configured to exhibitasymmetric characteristics and configured to switch from a firstpolarization state to a second polarization state responsive to exposureto a first bias voltage and configured to change from the secondpolarization state to the first polarization state responsive toexposure to a negative bias voltage having a different magnitude thanthe positive bias voltage.
 2. The semiconductor device of claim 1,wherein the ferroelectric material defines a portion of a capacitor. 3.The semiconductor device of claim 2, wherein the capacitor comprises atop electrode over the ferroelectric material and a bottom electrodeunder the ferroelectric material.
 4. The semiconductor device of claim3, wherein one of the top electrode and the bottom electrode comprisestitanium aluminum nitride.
 5. The semiconductor device of claim 4,wherein the other of the top electrode and the bottom electrodecomprises titanium nitride.
 6. The semiconductor device of claim 3,wherein one of the top electrode and the bottom electrode comprisestitanium, titanium nitride, titanium aluminum nitride, tantalum nitride,platinum, or combinations thereof.
 7. A memory cell, comprising: aferroelectric material configured to exhibit a switchable polarizationresponsive to exposure to a switching voltage, the ferroelectricmaterial configured to switch from a first polarization to a secondpolarization responsive to exposure to a first switching voltage andconfigured to switch from the second polarization to the firstpolarization responsive to exposure to a second switching voltage havinga different absolute value than an absolute value of the first switchingvoltage.
 8. The memory cell of claim 7, further comprising aninterfacial material comprising one of aluminum nitride and titaniumoxide between the ferroelectric material and an electrode comprising oneof titanium aluminum nitride and titanium nitride
 9. A semiconductordevice, comprising: at least one memory cell, the at least one memorycell comprising: a first electrode over a substrate; a ferroelectricmaterial over the first electrode, the ferroelectric material comprisinghafnium oxide, zirconium oxide, or a combination thereof and configuredto exhibit asymmetric switching characteristics; and a second electrodeover the ferroelectric material.
 10. The semiconductor device of claim9, wherein the ferroelectric material comprises hafnium oxide.
 11. Thesemiconductor device of claim 9, wherein the ferroelectric materialcomprises zirconium oxide.
 12. The semiconductor device of claim 9,wherein the ferroelectric material comprises one or more dopantsselected from the group consisting of silicon, aluminum, zirconium,magnesium, strontium, gadolinium, yttrium, and a rare earth element. 13.The semiconductor device of claim 9, wherein the first electrodecomprises a different material than the second electrode.
 14. Thesemiconductor device of claim 9, wherein the first electrode has adifferent thickness than the second electrode.
 15. The semiconductordevice of claim 9, further comprising at least one interfacial materialbetween the first electrode and the ferroelectric material or betweenthe ferroelectric material and the second electrode.
 16. Thesemiconductor device of claim 15, wherein the interfacial materialcomprises aluminum nitride.
 17. The semiconductor device of claim 15,wherein the interfacial material comprises titanium oxide.
 18. Thesemiconductor device of claim 15, wherein the interfacial materialcomprises a non-conductive dielectric material.
 19. The semiconductordevice of claim 9, further comprising an interfacial material betweenthe first electrode and the ferroelectric material and anotherinterfacial material between the ferroelectric material and the secondelectrode.
 20. The semiconductor device of claim 19, wherein the anotherinterfacial material comprises a different material than the interfacialmaterial.